The present invention relates to the field of air/fuel ratio control devices for internal combustion engines such as those used for automotive vehicles, and more particularly relates to the field of such air/fuel ratio control devices for engines which are equipped with carburetors in their intake systems and three way catalytic converters in their exhaust systems.
Three way catalytic converters for internal combustion engines are per se well known in various different forms. Such a three way catalytic converter is capable of converting HC, CO, and other products of incomplete combustion in the hot exhaust gases of the internal combustion engine into harmless end products by an oxidizing reaction, and also of simultaneously converting nitrogen oxides (so called NOx) in the exhaust gases into harmless end products by a reducing reaction, provided that the air/fuel ratio of the exhaust gases passing into said catalytic converter is maintained within a rather narrow range about the stoichiometric condition. If, however, the air/fuel ratio of the exhaust gases passing into said catalytic converter wanders towards the lean side stoichiometric, then although the above detailed oxidizing reaction for converting HC, CO, and other products of imcomplete combustion in the hot exhaust gases of the internal combustion engine into harmless end products continues, the reducing reaction for converting nitrogen oxides in the exhaust gases into harmless end products will substantially cease; and, if the air/fuel ratio of the exhaust gases passing into said catalytic converter wanders towards the rich side of stoichiometric, then although the reducing reaction for converting nitrogen oxides in the exhaust gases of the internal combustion engine into harmless end products continues, the oxidizing reaction for converting HC, CO, and other products of incomplete combustion in the hot exhaust gases into harmless end products will substantially cease.
It is possible to control the air/fuel ratio of the exhaust gases passing into the three way catalytic converter within a narrow range about the stoichiometric condition by controlling the air/fuel ratio of the air-fuel mixture being supplied to the internal combustion engine through its intake system within a narrow range about the stoichiometric condition, and therefore conventionally many different sorts of fuel/air ratio control systems have heretofore been proposed which have as their goal maintaining the air/fuel ratio of the air-fuel mixture being supplied to the internal combustion engine close to the stoichiometric condition.
A typical such prior art system has an oxygen sensor fitted to the exhaust manifold of the internal combustion engine, upstream of the three way catalytic converter, so as to sense the presence of oxygen in the exhaust gases therein. The signal from this oxygen sensor is then sent to a device which provides extra air into the intake system of the engine. In this case, the basic air/fuel ratio of the air-fuel mixture provided by the carburetor of the internal combustion engine is set to be rather on the rich side of stoichiometric, and thus by addition of a proper amount of extra air to the intake system the air/fuel ratio of the air-fuel mixture provided to the internal combustion engine may be controlled to be substantially the stoichiometric air/fuel ratio. Conventionally, the extra air can either be added directly into the intake manifold of the engine, downstream of the carburetor; or can be provided into a passage of the carburetor, as an additional amount of bleed air to be mixed with the fuel being provided by the carburetor, in a per se well known fashion. In either case, by feedback control performed by the extra air control device based upon the signal from the oxygen sensor, the air/fuel ratio of the air-fuel mixture provided into the cylinders of the internal combustion engine can be satisfactorily controlled to be substantially the stoichiometric air/fuel ratio, and thereby the air/fuel ratio of the exhaust gases passing into the three way catalytic converter can be satisfactorily maintained within a narrow range about the stoichiometric condition.
This kind of prior art feedback system is effective, and presents no problems for drivability of the vehicle incorporating the internal combustion engine under the engine load condition; but it is not satisfactory for engine idling operation. In fact, such a feedback system as outlined above causes surging and stumbling of the internal combustion engine to occur during idling, and stable idling operation becomes quite impossible.
In the prior art, a system that has been employed to overcome this problem has been developed as follows. Based upon the realization that during idling operation the production of nitrogen oxides by the internal combustion engine is not very considerable, and as a practical matter only the production of HC, CO, and other residues of incomplete combustion presents a major threat to the cleanliness of the atmosphere, it has been conceived of to operate the engine during engine idling condition by supplying thereto via the intake system an air-fuel mixture of air/fuel ratio substantially richer than stoichiometric, and then to inject a substantial amount of secondary air into the exhaust system of the engine, upstream of the three way catalytic converter, in sufficient amount to render the air/fuel ratio of the exhaust gases definitely leaner than stoichiometric at the time that the exhaust gases enter the three way catalytic converter. As a result, as noted above, the oxidizing reaction for converting HC, CO, and other products of incomplete combustion in the hot exhaust gases of the internal combustion engine into harmless end products continues satisfactorily, and although the reducing reaction for converting nitrogen oxides in the exhaust gases into harmless end products will substantially cease, this will not provide any great problem in practice, since as explained above the amounts of nitrogen oxides currently being produced are rather small. Further, because the air/fuel ratio of the idling air-fuel mixture being supplied to the internal combustion engine is substantially richer than stoichiometric, stumbling, surging, stalling, and irregular operation of the internal combustion engine during idling are substantially prevented.
A problem that has arisen with this prior art concept, in adapting it to actual automobiles of the sort that are being produced nowadays, is that it is common at the present time for carburetors of internal combustion engines for automobiles to be provided with so called idle up devices, which increase the idling speed of the internal combustion engine in response to increased idling load on the engine. For instance, conventionally and commonly engine idle up is performed when an air conditioner compressor is required to be operated during engine idling operation. Various other factors may also cause engine idle up to be performed, such as the operation of a power steering pump, or the like. The adaptation of the above concept of air/fuel ratio control to these cases has not been straightforward.